Help! Emails just sit in outbox and won't send

Please somebody help me. I'm using a macbook pro with OS X Yosemite. A while ago my emails stopped sending. After having a look in settings it looked like the account was offline. In the end I deleted the whole account from mail and then added it again. It has stopped saying it's offline now. But now when I try to send an email it no longer comes up with an error message, but just puts the email in my outbox where it stays. If I open it up and try to send it again literally nothing happens. It's literally driving me mad. It's a yahoo email account if that makes any difference. Please somebody help me.